1. Field of the Invention
The present invention relates to an optical pickup which is a device for recording and regenerating light information and, more particularly to a hologram unit of an optical pickup whose hologram plate is attachable thereto in a precise and prompt manner and a method of attaching the hologram plate by using the hologram unit.
2. Discussion of Related Art
An optical pickup is generally used as a device for reading out light information from an optical disk. Among various optical pickups of such a performance, a hologram unit which employs a hologram plate is illustrated in FIG. 1. The hologram unit largely includes a light source for reading out light information recorded on an optical disk 1 and an output device for providing a servo signal and a light information signal which drives an object lens 2 so as to read out the light information in a more accurate manner.
FIG. 2 shows a detailed construction of the hologram unit, which comprises a laser diode 3 (hereinafter, referred to as an "LD") for regularly generating a given front and back light, a monitor photodiode 4 (hereinafter, referred to as an "MPD") receiving the back light of the LD 3 and controlling the LD 3 to regularly emit a light beam, a multi-split photodiode 5 (hereinafter, referred to as a "PD") receiving the light information from the optical disk 1 and processing its corresponding signal, a stem 6 for supporting these photoelectric elements 3, 4 and 5, a protective cap 7 for safely covering the photoelectric elements, and a hologram plate 8 provided on the top surface of the cap 7 so as to transmit and diffract the light of the LD 3.
In the operation of this optical pickup, a driving section (not shown) drives the LD 3 to generate a front light to the hologram plate 8 and a back light to the PD 5.
The light beam launched on the hologram plate 8 is transmitted to the optical disk 1 via the object lens 2 so as to read out the light information of the optical disk 1. In addition, the light beam transmitted to the MPD 4 is converted to an electric signal, which is then used in the driving section (not shown) for controlling the LD 3 to regularly generate a light. The light information reflected from the optical disk 1 is fed back to the hologram plate 8 and refracted to the PD 5, where the refracted light is properly processed and reproduced.
The performance of this optical pickup depends on the precise attachment of the hologram plate 8, and as shown in FIG. 3, the hologram plate 8 forms a hologram 9 in the center of a perfect square thereon and, at the edges of the hologram 9, has four marks 10 whose extension lines altogether form a right cross mark.
According to the prior art, as shown in FIGS. 3 and 4, the radiant point 3a of the LD 3 is focused on the central point of the cross mark of a monitor 12 preadjusted by an observation microscopic lens 11, and the microscopic lens 11 is then moved right over the cross mark 10 on the hologram plate 8.
After the right cross mark (+) shown through the microscopic lens 11 is adjusted to the cross mark 10 on the hologram plate 8, the hologram plate 8 is attached to a predetermined region of the cap 7. In other words, the right cross mark (+) indicated by the radiant point 3a of the LD 3 is adjusted to the mark 10 on the hologram plate 8 by moving the microscopic lens 11 up and down. Then, the hologram plate 8 is attached at the point where the two marks meet.
Such an adjustment of the two marks and the subsequent attachment of the hologram plate 8 is attainable when the optical axis 8a passing the radiant point 3a of the LD 3 and the central point of the hologram plate 8 is in accordance with the optical axis 11a of the microscopic lens 11, as illustrated in FIG. 4.
In a reverse bias, if the LD 3 is not accurately aligned with the hologram plate 8, the optical axis 11a cannot meet the connection line of the radiant point 3 of the LD 3 and the optical axis 8a only by moving the microscopic lens 11 up and down, as shown in FIG. 5. Therefore, there may occur an error in the arrangement of components because the reality is different from the correlation between the image 3a' of the LD radiant point 3a shown on the predetermined cross mark (+) of the monitor 12 and the images 9' and 10' of the marks displayed on the hologram plate 8, as illustrated in FIGS. 6A and 6B.
In the prior art, the hologram plate can be precisely attached by moving the microscopic lens 11 up and down once or more when the above two light axes perfectly meet each other. However, a problem may be encountered with a false alignment of the two axes, so that it is necessary to additionally adjust the axes to be in accordance with each other and it leads to the deterioration of the hologram.